This invention generally relates to a high efficiency power amplifier system. More specifically, this invention relates to an amplifier system with load control for efficient amplification in a linear modulation scheme.
The rapid and great increase in the cellular phone use has resulted in a shortage of frequency spectrum for handling all of the cellular customers. This shortage has prompted a migration towards more spectrally efficient, digital modulation schemes that have higher user capacities. Examples of such modulation schemes include Interim Standard (IS)-136 Time Division Multiple Access (TDMA), Personal Digital Cellular, IS-95 based Code Division Multiple Access, Global System for Mobile communications (GSM) and TDMA Edge. In addition, newer and even higher data rate capacity systems are being proposed and developed for third generation cellular systems.
The digital cellular systems usually require linear modulation, and while the linear modulation does facilitate higher system capacity, it also results in lower efficiency power amplifiers. Power amplifiers continue to consume a significant percentage of the overall current drain of the portable radiotelephones used in cellular communication systems. The higher the current drain, the less talk time and stand-by time that is available, since the battery for the portable radiotelephone will be drained faster. Thus, the efficiency of the power amplifiers used in portable radiotelephones is a critical parameter; the higher the efficiency, the less current the power amplifier consumes and the more talk/stand-by time is available for the portable radiotelephone. Several techniques have been proposed to improve the efficiency of power amplifiers linear modulation systems.
One technique for improving the efficiency of power amplifiers is Envelope Elimination and Restoration (EER). EER was first proposed by Leonard Kahn in 1952 (July Proceedings of the I.R.E. page 803-806). An example block diagram of a power amplifier (PA) EER system is depicted in FIG. 1. The PA EER system 100 includes a limiter 2 coupled to a power amplifier 4, and the power amplifier 4 coupled to antenna 14. The PA EER system 100 further includes a voltage supply control circuit 6 coupled to the power amplifier 4 through a filter 8.
In a many digital cellular systems, the information bearing radio frequency signal contains both amplitude modulation (AM) and phase modulation (PM) components. An RF signal with AM and PM components appearing at input 12 is processed by the limiter 2. The limiter 2 removes all AM information and passes a substantially constant envelope signal to the power amplifier 4. The limiter can be a simple limiting RF amplifier.
In addition, an envelope signal appearing at input 16 contains information about the RF signal envelope of the RF signal that appears at input 12. The envelope signal is applied to the voltage supply control circuit 6. Finally, a substantially constant supply voltage is applied to the voltage supply control circuit 6 at input 10. The voltage supply control circuit 6 along with filter 8 comprises a switching power supply that modulates the supply voltage appearing at input 10 in response to the envelope signal appearing at input 16. The voltage supply control circuit 6 and the filter 8 thus supply a varying voltage signal on line 18 that varies according to the AM envelope of the RF signal originally appearing at input 12. By modulating the supply voltage to the power amplifier, the desired AM envelope is impressed on the output signal of the power amplifier, and the resultant signal with the restored AM envelope is transmitted through antenna 14. Since the AM of the RF input signal appearing at input 12 is removed, the PA EER system 100 allows the power amplifier 4 to operate as a very efficient class C amplifier. All of the AM information is impressed upon the output signal of the power amplifier by the changes in the power amplifier supply voltage appearing on line 18.
There are several difficulties and shortcomings of the PA EER system 100. First, limiters that sufficiently remove the AM of the RF input signal are difficult to realize. The difficulty increases as the operating frequency increases. Second, the voltage supply control circuit 6 and the filter 8 are substantially a switching power supply. These circuits typically consume much power that ultimately factors into and subtracts from the overall efficiency of the PA EER system. Third, it is difficult to develop a voltage supply control circuit 6 that meets the bandwidth requirements necessary for the voltage supply control circuit 6 to follow the AM evelope of wide band systems such as code division multiple access (CDMA) cellular telephone systems. For example, the envelope signal appearing at input 16 is generated by decomposing the RF signal appearing at input 12 into separate AM and PM signals. The decomposed AM signal has significantly higher bandwidth than the composite signal, and the voltage supply control must operate at the bandwidth of the decomposed AM signal. For many of the third generation cellular systems, switching speeds in the range of approximately 20 MHz could be required for such a PA EER system. And finally, the PA EER system 100 is expensive to produce. The filter 8 requires several components including at least one large inductor. These parts tend to add considerable cost and size to the portable radiotelephone, and since portable radiotelephones is many times a commodity product, price is a critical parameter.
Another technique for improving the efficiency of power amplifiers is called Envelope Following (EF). A simplified block diagram of an EF system is shown in FIG. 2. The PA EF system 200 includes a power amplifier 30 coupled to antenna 40 in the RF path and a voltage supply control circuit 32 coupled through filter 34 to the power amplifier 30 on the supply path.
An RF signal having AM and PM modulation components appearing at input 38 is applied to the power amplifier 30. In addition, an envelope signal containing information about the AM envelope of the RF signal at input 38 is applied through input 42 to the voltage supply control circuit 32. Once again, the voltage supply control circuit 32 and the filter 34 are in essence a switching power supply. A substantially constant supply voltage is applied to the voltage supply control circuit 32 through input 36.
In this system, the AM is not removed from the RF signal appearing at input 38. Rather, the supply voltage supplied to the PA 30 through line 44 is reduced or increased responsive to the amplitude of the AM envelope. The supply voltage applied to the power amplifier 30 thus follows the AM envelope of the RF signal applied to the power amplifier 30. By modulating the supply voltage to the power amplifier 30 to follow the AM envelope, less power is consumed overall. For example, when the envelope is at a peak, the supply voltage to the power amplifier 30 is increased, but when the envelope is at a minimum, the supply voltage decreases, thereby saving power. This significantly increases the efficiency of the power amplifier 30.
Since the actual AM on the output signal produced by the power amplifier 30 does not originate in the voltage supply control circuit 32, some of the bandwidth requirements on the voltage supply control circuit 32 and filter 34 are reduced relative to the PA EER 100. This system also eliminates the need for a high frequency limiter. However, PA EF system 200 still requires a voltage supply control circuit 32 and filter 34, which adds significant cost and size to a portable radiotelephone.
Accordingly, there is a need for power amplifier systems operable in linear modulation systems. There is a further need for these linear power amplifier systems to be efficient so as to minimize current drain for operation in portable radiotelephones. There is a further need to minimize the cost of producing linear power amplifier systems in order to make the portable radiotelephones that utilize the power amplifier systems price competitive.